docs: def on_touch_down(self, touch):
          """If I'm the first card to collide this touch, grab it, store my
          metadata in its userdict, and store the relative coords upon
          me where the collision happened.
  
          """
          if not self.collide_point(*touch.pos):
              return
          if 'card' in touch.ud:
              return
          touch.grab(self)
          self.dragging = True
          touch.ud['card'] = self
          touch.ud['idx'] = self.idx
          touch.ud['deck'] = self.deck
          touch.ud['layout'] = self.parent
          self.collide_x = touch.x - self.x
          self.collide_y = touch.y - self.y